The present invention concerns an arrangement in a power saw to automatically release a safety brake which is arranged to stop the saw chain travelling around the cutter bar of the power saw.
When working with chain power saws it is possible to have the outer tip of the saw chain accidentally come into contact with a tree trunk or similar object, resulting in unexpected jerky movements of the saw. This is a well recognized risk, and to reduce the chance of bodily harm to the saw operator, a safety brake is normally arranged in the saw in such a manner as to become immediately operative upon occurrence of such jerky movements of the cutter bar.
Usually, the brake consists of a belt travelling around a drum and connected to a pre-tensioned spring via a blocking mechanism. In addition, a protective yoke is provided in front of one of the handles of the power saw and connected with said blocking mechanism. When jerky movements occur that are so strong that the operator loses his grip on the saw handle and his arm strikes the protective yoke, the latter triggers the brake to stop the power saw chain.
A power saw constructed in this manner is not, however, completely satisfactory from a safety point of view because to trigger the brake it is necessary that the saw operator somehow engage the protective yoke. This is not always the case. Should the saw operator, despite a heavy jerk of the cutter bar, be able to retain his grip on the handle, the cutter bar may instead hit a person standing beside the operator. A jerky movement may also occur when the saw is in felling position, in which case one of the operator's hands is not positioned between the protective yoke and the handle.
One way of eliminating the above problem is to make use of the relative movement between the handle, yoke and the body of the saw that is possible in power saws built with a vibration-dampening member. On account of the relative movement which arises at the occurrence of a strong jerky movement, the brake mechanism may be triggered off via actuating means of some kind. Triggering mechanisms built on this principle are not, however, completely reliable because the vibration-dampening members which consist of blocks or insets of rubber generate relative movements of varying amplitude depending on the surrounding temperature. Should the temperature decrease radically from one day to the next the modulus of elasticity of the members will also change excessively, which may lead to non-actuation of the brake at the occurrence of jerky movement.
In accordance with another technique which eliminates the need for displacement of the protective yoke to stop a jerky movement, the cutter bar is arranged to perform a short pivotal movement relative to the body of the saw. In addition, the cutter bar is connected to a spring means which upon a jerky movement of the cutter bar dampens this movement and thus enables the saw operator to retain his grip on the power saw. However, this device does not bring the saw chain to a standstill. If it were combined with a brake of the kind outlined above, the spring means would to some extent counteract and thus delay the triggering of the brake.